Sub: eliminate calls to manipulate _writes_enabled

ArduSub/Log.cpp

@@ -482,9 +482,6 @@ void Sub::start_logging()
     if (!DataFlash.logging_started()) {
         DataFlash.StartNewLog();
     }
-
-    // enable writes
-    DataFlash.EnableWrites(true);
 }
 
 void Sub::log_init(void)

ArduSub/motors.cpp

@@ -120,12 +120,6 @@ void Sub::init_disarm_motors()
 
     DataFlash_Class::instance()->set_vehicle_armed(false);
 
-    if (DataFlash.log_while_disarmed()) {
-        start_logging(); // create a new log if necessary
-    } else {
-        DataFlash.EnableWrites(false); // suspend logging
-    }
-
     // disable gps velocity based centrefugal force compensation
     ahrs.set_correct_centrifugal(false);
     hal.util->set_soft_armed(false);